1. Field of the Invention
The present invention relates to the field of the photoelectron technique, and more specifically, it relates to a hybrid coupling structure of a short range surface plasmon polariton and a conventional dielectric waveguide and a coupling structure of a long range surface plasmon polariton and a dielectric waveguide, as well as a refractive index sensor, a TM polarization modulator etc. based on the structures.
2. Description of the Background Art
A surface plasmon polariton (SPP) is an electromagnetic field propagating along the interface between a metal and a dielectric. As shown in FIG. 1, 1 denotes a metal (or a mixture of a metal and a dielectric), 2 denotes dielectrics around the metal and numeral 3 denotes surface plasmon polaritons on interfacial portions, and coupling develops in the SPPs on the upper and lower surfaces when the metal is sufficiently thin, to form a symmetric mode, i.e., a long range surface plasmon polariton 4, or an antisymmetric mode, i.e., a short range surface plasmon polariton 5.
An SPP is a surface wave, and the electromagnetic field energy thereof concentrates in the vicinity of the interface between a metal and a dielectric, while the amplitude of the electromagnetic field is exponentially attenuated following the distance separating from the interface in the dielectric. When a metal film is relatively thin, plasmon polaritons on the upper and lower surfaces develop coupling, and form two types of new surface plasmon polariton modes.
As shown in FIG. 2, one of the modes is the symmetric mode, whose mode field is mostly distributed in the dielectrics other than the metal, has relatively small propagation loss and is propagable over a relatively long distance along a metal film, and this type of mode is referred to as a long range surface plasmon polariton (LRSPP).
Another mode is the antisymmetric mode, which is more approximate to the metal, has relatively large propagation loss and is only propagable over an extremely short distance along the metal film, and hence the same is referred to as a short range surface plasmon polariton (SRSPP).
Thus, a thin metal or a metal strip becomes a surface plasmon waveguide (SPP waveguide). When propagating a long range or short range surface plasmon polariton, the same is referred to as a long range or short range surface plasmon waveguide.
When the distance between a metal waveguide and a dielectric waveguide is sufficiently small, a conventional dielectric waveguide mode develops coupling with an SPP mode under constant conditions. An SRSPP has a characteristic of approximating a metal at a higher degree as compared with SPP and LRSPP modes, and hence such a new coupling phenomenon that this type of SRSPP hybrid-couples with a dielectric waveguide renders the coupling length and the size of a device shorter and smaller respectively, and has wide-ranging applications in the field of a highly integrated photon device and optical communication.
Further, the electromagnetic field energy of the SPP concentrates in the vicinity of the interface between the metal and the dielectric, and hence the electromagnetic field on the metal surface is extremely strong and extremely sensitive to the form of the surface, particularly a change in the refractive index, and has wide-ranging applications in the biosensor field.
However, a wave field of a the short range SPP wave approximates a metal surface at a higher degree as compared with a conventional SPP wave and a mode characteristic thereof is extremely sensitive to a refractive index change in a dielectric in an ultrathin range around a metal film, and hence an obvious change is caused in the coupling of a short range surface plasmon polariton mode and a conventional dielectric waveguide mode and a drastic change in output power of the dielectric waveguide is caused if the refractive index of an ultrathin layer material above the metal film changes (the most biological reactions belong to this type of ultrathin layer reaction). This provides a new way for high precision detection of the refractive index of the ultrathin layer material.
A conventional biosensor of a surface plasmon polariton requires separate components such as a prism and a rotation table, and hence the same has a large volume, is not only hard to control but also has low detection sensitivity with respect to an ultrathin layer material, is inferior in stability and requires a high cost, whereby spreading/application thereof is strictly limited.